Dairy Sheep Symposium - the Department of Animal Sciences ...

The milk of dairy cows (Losi et al. 1982) and sheep (Casoli et al. 1992) suffering from
mastitis does not clot and is not suitable for cheese production.
Somatic cells increase dramatically with any inflammatory or pathological process affecting
the mammary gland (Morgante et al. 1994). A high somatic cell count results in changes in the
composition of milk, with a reduction in fat, casein, and total solids and an increase in total
nitrogen, non-protein nitrogen, whey proteins (Duranti and Casoli 1991; Pulina et al. 1991;
Bufano et al. 1994). Milk minerals have also been reported to change, with increased chloride
and decreased phosphate, citric acid, potassium and magnesium, and a consequent increase in
pH (Harmon 1995). These changes are also accompanied by a worsening of the clotting parameters
such as renneting time, rate of curd formation and curd consistency and a reduced cheese
yield due to an increased loss of fat in the whey (Duranti and Casoli 1991; Pirisi et al. 1994).
High somatic cell counts in cow’s milk have been associated with problems in the quality of
cheese (Politis and Ng-Kway-Hang 1988), but in the only study conducted on the effect of high
somatic cell counts on sheep milk cheese no change in quality was detected in the cheese after 2
months maturation (Pirisi et al. 1994). Therefore it appears that more research is required to
establish if high somatic cell counts are of relevance for manufacture of sheep milk cheese.
Microbial cell count
The microbial cell count in milk is due to the presence of microorganisms of which some
(Lactobacillus spp, Lactococcus spp, Streptococcus spp) can be advantageous for transformation
into cheeses, while others can cause human diseases (eg Listeria, Salmonella, Brucella) or
problems in the maturation of the dairy products (eg Enterobacteriaceae, Coliforms,
Psychrotrophs, Clostridium spp) (Fatichenti and Farris 1973).
Psychrotrophic bacteria such as Pseudomonas, Leuconostoc and Micrococcus thrive at
temperatures below 7°C and produce lipolytic and proteolytic enzymes which destabilise the
casein micelles and alter the clotting properties of milk (Moquot and Auclair 1967; Durr 1974;
Milliére and Veillét-Poncét 1979; Bloquel and Veillét-Poncét 1980; Juven et al. 1981; Mottar
1984; Nuñez et al. 1989; Uceda et al. 1994a; 1994b).
Enterobacteriaceae and coliforms are generally of faecal origin and ferment the lactose in the
cheeses producing large quantities of gas, causing early spoilage of the cheese (Gaya et al.
1987).
In Europe, sheep milk can be processed into cheese if its bacterial count is less that 1 million/
mL provided the milk is pasteurized prior to processing. For cheese making from raw milk the
bacterial count must be less than 500,000/mL, but there are no rules concerning somatic cell
counts for the processing of sheep milk (European Union Directive 46-47/1992).
Since in Australia and New Zealand sheep milk is listed as a non defined product, there are
no precise law requirements for its microbiological quality, but it can be assumed that regulations
applying to cow’s milk could be extended to sheep milk. In Australia good quality cow’s
milk should have a bacterial count of less that 50,000/mL, but maximum allowed counts vary
according to the Dairy Industry Acts of the different states. These limits often are reflected in
payment schemes, through premiums or penalties, aimed at encouraging the production of high
quality milk. The Australian law also requires that all dairy products must be produced from
pasteurized milk. This is not strictly required in the Mediterranean countries, where some hard
cheeses are produced from raw milk.
The occurrence of undesirable bacteria can be avoided by applying correct milking and milk
handling procedures.